Surfactants are amphiphilic molecules which generally contain a hydrophilic and hydrophobic domain. Four classifications of surfactants exist based on the nature of the hydrophilic group including: 1) nonionic (neutral charge); 2) anionic (negative charge); 3) cationic (positive charge); and 4) amphoteric or zwitterionic where both a positively and negatively charged group are positioned typically in close proximity at the hydrophilic end. Charged groups on surfactants can be characterized by their pKa. When molecules are suspended in solutions which have solution pH at the pKa of the group, the group is neutral. At solution pH values above the pKa, the group is negatively charged; while at solution pH values below the pKa, the group is positively charged.
Examples of surfactants are soaps (typically sodium stearate, comprising about 50% of the yearly production of surfactants, about 15 million tons/year) which are able to exist in aqueous media through the formation of micelles where the hydrophobic and hydrophilic ends of the molecules align and form a generally spherical construct where the hydrophobic ends are located in the interior and exclude water. Importantly, micelles are dynamic structures which can be disrupted via mechanical processes like shear though agitation or extrusion then reform creating stable suspensions. Surfactants can coat materials of different phases to create what are known as emulsions.
Surfactants can be natural or synthetic. Synthetic surfactants include but are not limited to diacetyl tartrate esters of monoglycerides [DATEM], acetylated monoglyceride [AcMG], lactylated monoglyceride [LacMG], and propylene glycol monoester [PGME]), sorbitan derivatives (e.g., sorbitan monostearate, sorbitan monooleate and sorbitan tristearate), polyhydric emulsifiers (e.g., sucrose esters and poly glycerol esters like polyoxyethylene (20) sorbitan monostearate [Polysorbate 60], polyoxyethylene (20) sorbitan tristearate [Polysorbate 65], and poly glycerol monostearate.
Natural surfactants include lipopeptides and lipoproteins, glycolipids, phospholipids, fatty acids and polymeric surfactants. Many can be used in food production. Some specific examples of anionic fatty acid food surfactants include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid and stearic acid. A specific example of a cationic food surfactant is lauric arginate which also has anti-microbial properties.
Natural, biologically derived food surfactants have an advantage, as they are environmentally friendly, edible and generally safe in virtually any application. Those already used in food production also have the advantage of being readily available in volume quantities and generally low in cost.
There are many applications of surfactants, including detergents, fabric softeners, emulsions, paints, adhesives, inks, waxes, de-inking of recycled papers, enzymatic processes, laxatives, agrochemical formulations, some herbicides and insecticides, pollution remediation, stabilization of nanomaterials such as quantum dots, biocides and sanitizers, cosmetics, shampoos, hair conditioners, toothpastes, pharmaceuticals, drug delivery, food compositions, some spermicides, liquid drag reducing agents for pipelines, oil recovery, and many others. The many diverse applications of surfactants arise from the important function they can perform: compatibilizing an interface between a polar material and non-polar material.
Multi-surfactant compositions have been implemented previously. Broze et al. (U.S. Pat. No. 4,622,173) demonstrated improved liquid laundry detergents containing three surfactants. Specifically Broze, et al. related to laundry detergents with improved detergency obtained from a mixture of an acid-terminated non-ionic surfactant with a quaternary ammonium salt surfactant. Mehreteab et al. (U.S. Pat. No. 5,472,455) used mixtures of anionic and cationic surfactants to improve the removal of oily stains from fabrics. Thunemann et al. (U.S. Pat. No. 6,486,245) disclosed a coating composition based on a complex of polyelectrolytes and oppositely charged surfactants. The surfactants contain fluorine bonded covalently to carbon atoms. The coating material imparts oleophobic and/or hydrophobic properties to various surfaces. However, the multi-surfactant systems disclosed in these patents are not engineered to respond or change dynamically to the environment in which they are used, i.e., change in the degree of polarity of the surfactant system or change the size or structure of any formed micelles as a result of changes in ionic strength or solution pH.
Chen et al. (U.S. Pat. No. 8,211,414) disclosed water soluble polymer complexes with surfactants. Specifically they disclosed complexes including a polymer and a surfactant formed by polymerizing a monomer mixture containing: (A) acid functional monomers at least partially neutralized with one or more amines according to one or more of formulas (I) through (IV): R1—NR2R3 (I) R1—N+R2R3R7X− (II) R4—C(O)—NR5—R6—NR2R3 (III) R4—C(O)—NR5—R6—N+R2R3R7X− (IV) where R1 and R4 are independently C8-C24 linear, branched or cyclic alkyl, aryl, alkenyl, aralkyl or aralkyl; R2, R3 and R5 are independently H or C1-C6 linear, branched or cyclic alkyl; R6 is C1-C24 linear, branched or cyclic alkylene, arylene, alkenylene, aralkylene or aralkylene, R7 is H, C1-C12 linear, branched or cyclic alkylene, arylene, alkenylene, aralkylene or aralkylene, and X is a halide, a sulfate or a sulfonate; (B) one or more cationic monomers; and optionally (C) one or more other monomers. Although a polymer-surfactant complex was formed, the properties or behavior of the complex or surfactant were not engineered to respond to or change dynamically to the environment in which they are used.
In addition, dispersion of cellulose nanoparticles in non-polar matrices using a variety of surfactants has been explored previously, and a few results involve biologically based surfactants. In these cases, however, the surfactant was passive and either simply adsorbed onto the surface through, for example, electrostatic interactions, or covalently coupled to the surface via a crosslinking chemistry.
Accordingly, a continuing need exists for active, environmentally responsive multi-surfactant systems and for compatibilizing disparate materials particularly at the interface thereof.